Earth working bit assemblies utilizing hardened insert cutting edges are commonly employed in various types of earth grading applications. Many bit assemblies are used for attachment on, but not limited to, motor grader moldboards.
Due to the various earth conditions that motor grader bit assemblies are subjected to, many modifications of the motor grader bit assemblies have been developed. For the grading of soft earth formations, a single, replaceable cutting edge bolted to the lower edge of the moldboard is adequate. But when the dressing of hard impacted, rough, and abrasive earth formations is needed, the continuous steel edge mentioned above is quite inadequate.
It has been widely known that to extend the life of the cutting edge one must merely add hardened inserts, usually of tungsten carbide. One of the known grader bit assemblies is shown in U.S. Pat. No. 3,529,677 to E. W. Stephenson issued on Sept. 22, 1970. In this arrangement a plurality of carbide inserts were brazed side by side in a continuous slot all along the bottom surface of the cutting edge attachment. This grader bit assembly prolonged the life of the cutting edge, but many problems still remained with its utilization. In hard, compacted earth formations, the continuous cutting edge prevented the blade from penetrating the upper portion of the ground and the blade tended to ineffectively slide along the top portion of the earth surface.
Another disadvantage of the continuous edge became apparent in applications where the earth surface was highly abrasive and rock impacted. The continuous cutting edge, formed by the hardened inserts, tended to crack and chip due to high forces imparted onto the edge by the hard, rock impacted earth surface. Many of the carbide inserts tended to dislodge, causing an irregular edge and premature loss of the expensive carbide. Once the carbide chipped or became dislodged, the steel edge wore at an accelerated rate.
It has been shown that a discontinuous cutting edge is far superior in performance on hard surfaces than is a continuous edge. When the discontinuous cutting edge was constructed of steel, problems occurred. Due to the high downward pressure needed to penetrate the hard upper earth surface, tooth breakage often occurred. This resulted in significant downtime and cost. Another problem occurred when sudden penetration of the hard earth surface took place due to the high downward pressure applied to the grader blade. When sudden penetration occurred, usually at irregular intervals, trenches were imparted into the graded surface, creating pockets of potholes and surface irregularities.
U.S. Pat. No. 4,753,299 to Thomas A. Meyers issued on June 28, 1988, proposed to alleviate the above problems associated with grading of hard earth surfaces and provide a blade bit assembly that reduced downtime associated with broken and worn carbide cutting edges and the replacement of carbide inserts. The invention provided a moldboard attachment that has a plurality of pick assemblies attached thereto. The pick assemblies each has a singular, thin rectangular carbide insert brazed to the leading edge of the pick assembly. The moldboard pick assembly provides an adequate grading action on harsh earth surfaces, and provides for replacement of individual picks when wear and breakage occur, but many difficulties are still associated with this arrangement.
The main disadvantage of the above arrangement is in the nature and design characteristics of the pick assembly. Because the insert is located on the leading face of the pick body, it encounters high impact forces and high forces urging the bit assembly downward. These forces tend to crack and break the carbide insert. This leads to premature tool wear out, inefficient loss of carbide, and accelerated wear of the leading face.
Due to the design and the properties of the carbide insert, cracking and breakage of the insert frequently occurs. The thin carbide insert is brazed to the leading face of the pick assembly. Due to the differing coefficients of thermal expansion, the steel expands and contracts more rapidly than does the thin rectangular carbide insert. This results in the steel imparting high residual stresses onto the adjacent carbide insert due to the differing rates of expansion and contraction. The problem is further accentuated by the fact that the arrangement uses a thin rectangular piece of carbide. The combination of the thin carbide (small cross sectional area between the front and rear surfaces) and the differing coefficients of thermal expansion results in severe cases of cracking and breakage of the carbide inserts when used in applications on hard and/or rock impacted surfaces.
Another disadvantage of the above arrangement is apparent when the phenomena of "rolling" takes place. As the moldboard pick assembly grades a rough, abrasive surface, the inserts actually shave the material from the earth's surface. As the material is shaved, it rolls under the pick assembly base and wears away the steel support behind the insert. The steel support acts as a reinforcing support member. Without the steel support behind the insert, large portions of the carbide insert breaks off as a result of the forces encountered while grading.
To alleviate the rolling phenomena a second rectangular carbide insert was added at a spaced location behind the first insert and parallel thereto. The rolling phenomena now occurs behind both inserts, allowing both inserts to lose their needed steel support and subsequently causes both inserts to crack and break away. The addition of the second insert only increases the cost of the pick assembly due to the increased amount of carbide and the extra machining required to properly locate the second carbide insert.
The present invention is directed to overcoming one or more of the problems as set forth above.